#include"stdio.h"
#include"math.h"

int main()
{
	int a, b, c;
	while (scanf_s("%d%d%d", &a, &b, &c) == 3)
	{
		if (a <= 0 || b <= 0 || c <= 0 || a + b <= c || a + c <= b || b + c <= a)
		{
			printf("-1\n");
			continue;
		}
			double p = (a + b + c) / 2.0;
			double area = sqrt(p * (p - a) * (p - b) * (p - c));
			printf("%.2f\n", area);
	
		return 0;
	}
}
